To allow hypervisor drivers to assume that a lock driver impl
will be guaranteed to exist, provide a 'nop' impl that is
compiled into the library

* src/Makefile.am: Add nop driver
* src/locking/lock_driver_nop.c, src/locking/lock_driver_nop.h:
  Nop lock driver implementation
* src/locking/lock_manager.c: Enable direct access of 'nop'
  driver, instead of dlopen()ing it.